1. Field of the Invention
The present invention relates to a photonic microwave generation apparatus and method thereof by using, particularly, period-one nonlinear dynamics of semiconductor lasers.
2. Description of the Related Art
Due to the continuously increasing demand for transmission of massive data and high-definition videos, a considerable enhancement in the capacity of a communication system has become urgently important. For current wireless communication systems, the operating frequencies of the microwave carriers are mostly below 6 GHz, which limits the enhancement of the communication bandwidth and therefore restricts the capability of the current systems to support the continuously increasing requirement of communication capacity. If the frequencies of the microwave carriers can be significantly increased, the communication bandwidths and therefore the communication capacity provided by the wireless communication systems can be substantially improved.
Hence, telecommunication operators and manufacturers have proposed to adopt high-frequency microwave signals as carriers for the next-generation wireless communication systems (For example, Samsung and Nokia have proposed to use 28 and 70 GHz, respectively) in order to provide a communication bandwidth of 100 or even 1000 times more than that of the current systems. On the other hand, they have also proposed to adopt an architecture for the next-generation wireless access networks, which combines the wireless and fiber-optic communication systems, to enjoy the advantages both systems provide. In this manner, not only a considerably broader communication bandwidth is provided, but also a significantly wider communication coverage is feasible. Compared with the current wireless network architecture, more than 80% of signal processing functionalities will be moved from the remote base stations to the central offices in the next-generation network architecture. The most important signal processing functionalities include (1) the generation of high-frequency microwave signals and (2) the superposition of such microwave signals onto optical carriers for fiber distribution to remote base stations. As opposed to electronic microwave generation apparatuses that require two different steps to complete the aforementioned microwave generation and superposition, photonic microwave generation apparatuses need only one step based on an approximately all-optical approach to generate optically carried high-frequency microwave signals. In this manner, not only expensive electronic devices and equipment are much less needed, but also the limitation in generating high-frequency microwave signals due to the electronic bandwidth restriction can be mitigated.
Three commonly adopted photonic microwave generation apparatuses and methods for the generation of high-frequency microwave signals are briefly described as follows.
(1) Optical Heterodyning:
This is an all-optical method without the need of any electronic devices, which uses interference between two continuous-wave optical signals of different optical frequencies at a photodetector. The frequency of the generated microwave signals is broadly tunable up to the order of 1 or even 10 THz. However, since the two optical signals are generally not phase correlated to each other, the frequency of the generated microwave signals jitters significantly and the linewidth of the generated microwave signals is considerably broad, which are disadvantageous for practical applications. Therefore, an approach using an optical phase-locked loop is generally needed for this photonic microwave generation method to solve these two problems. However, the optical phase-locked loop itself is a very complicated electronic circuit requiring many high-frequency electronic components, which offsets the initial advantages this photonic microwave generation method provides.
(2) Optoelectronic Oscillator:
This method sends in a continuous-wave optical signal into an optoelectronic loop, which consists of an optical modulator, an optical delay line, a photodetector, a microwave amplifier, and a narrow bandpass filter, for the generation of high-frequency microwave signals. Even though the frequency of the generated microwave signals can be up to 75 GHz, the range of the tuning is limited. The advantages of this method include that the generated high-frequency microwave signals are extremely stable in their frequency and narrow in their linewidth, and that no electronic microwave signal generator is necessary. However, the disadvantages of this method include that many high-frequency electronic and photonic devices are required, and that the tuning range of the microwave frequency is restricted which limits the extent of the re-configurability of the method for different frequency requirements.
(3) Mode-Locked Semiconductor Laser
This method utilizes the interference between the longitudinal modes of a mode-locked semiconductor laser at a photodetector. Even though the system configuration is relatively simple, this method needs a very expensive and complex mode-locked semiconductor laser. In addition, the frequency of the generated microwave signals is only up to about 50 GHz and cannot be adjusted or tuned once the laser is given. The advantage of this method is that the generated microwave signals are stable in their frequency and narrow in their linewidth. However, the disadvantage of this method is that the microwave frequency cannot be tuned, which provide the method with no re-configurability for different frequency requirements.